With a traditional pulse-frequency modulated (PFM) controller, switching pulses with a fixed size are sent whenever the output voltage of the power supply drops below a defined threshold. The switching frequency will thus depend on the magnitude of the load (a heavier load will result in a higher pulse frequency). This method of control allows for a high efficiency to be achieved during light load conditions compared to pulse-width modulation (PWM) since a low pulse frequency will result in reduced switching losses.
Since the switching frequency of a traditional PFM controller depends on the magnitude of the load, the frequency can vary to a large degree. An undesirable condition can occur when the switching frequency enters a band that is audible to the human ear (20 kHz or less). When the frequency falls into this range, the switching action of the power supply can sometimes become audible. As a result, designers often need to sacrifice the high efficiency of PFM operation for PWM in order to avoid audible switching noise.
On the other hand, as the load becomes heavier, the switching frequency increases with a fixed pulse width. As a result, the switching losses increase proportionally to the load. Furthermore, if the controller is implemented digitally, more frequent PFM logic operations will result in lower efficiency in the controller.